Sound recording system



- Jan. 2o, 1942.

c. N.A BATSEL soUNo RECORDING SYSTEM Filed oct. 2o, i939 www PatentedJan. 20, 1942 SOUND RECORDING SYSTEM Cecil N. Batsel, North Hollywood,Calif., assignor to4 Radio Corporation of America, a. corporation of IDelaware Application october' 2o, 193,9, serial No. 300,401

9 Claims.

This invention relates to sound apparatus and particularlyv to a soundfilm recording system whereby greateri flexibility of recording andbetter' reproduction of the records made with the system are obtainable.i

vThe invention is particularly related to the variable area type ofsound recording whereby a trace or traces longitudinally of the filmsound track area separate a varying width opaque portion `from a varyingwidth transparent portion, the transverse variations of the tracevarying linearly with the amplitude of the signal being recorded. Thislinear relationship is` desirable to accurately record and reproduce thesound source without distortion. However, it is` well known that thesound track area of.' a vfilm is of limited Width andv that frequentlythe signal peaks are higher. than the upper limit of' the film. In suchinstances the peaks are cut off, producing square-topped areas betweenthe point where the signal Went beyond the film width `and againreturned thereto, and thus introducing a large amount of distortion. Inrecording practice, an operator attempts to control the signal level inthe electrical transmission circuit between the microphone and lightmodulator. He attempts to bring the highv peaky levels within the iilmrange by reducing the gain or increasing the attenuation in theelectrical? circuit. This operation is, of course, beneiicial but it isfrequently difficult for the operator to determine the exact point atwhich the-level will go beyond the film width. Furthermore, the changein gain of the circuit introduces varying amounts of distortion andsometimes eliminates or' at least reduces the dramatic effect'intended.It has also been found, in the reproduction of signals in theatres, thatthe average level of theatre reproduction is ,15 or 20 db. greaterthanthe original level, particularly in the case of dialogue recording. Atnormal speech'loudness, that is, the level at which speech is naturallylhead without artificial aid, considerable volume latitudev ispermissible without annoyance; When this average speech isreproducedscme 20 db. higher without compression of momentary peaks, theloudness at these-points causes extreme annoyance to the listener. Thiscondition is found theatre reverberation serve to heighten this' effect.These energy peaks are not necessarily expressive dynamics used bytheactor, butr may befcaused by lack of breath control or othervocal CIDdefects and the less-trained' the actor the more noticeable the defect;However, the dynamics of even the well trained are uncomfortablyexaggerated when reproduced at theatre loudness.

It is well known in the art that electronic. compressors have been usedto control the sound level in the electrical transmission circuitbetween themicrophonefand sound recorder, the high energy peaks being'automatically brought Within the iilm range. This, of course, requiresadditional electrical circuits in which distortion may' occur, while thecompression is generally effective over a considerable range of' thesignal, necessitating in many cases the use of.` eX- panders toreconvert the signal to normal. Such f a system is disclosedin U; S.Patent 2,136,723 of November 15, 1938. Curved recording apertures toproduce ygeneral compression, as shown in U. S. Patent 1,767,790 of June24, 1930, have also been employed, this type of system also requiringexpansion reconversion circuits to bring the signal to normal. Thepresent invention, therefore, is directed to a simple arrangementof arecording aperture whereby the necessary expressive dynamics are broughtwithin the film range with intelligibility and without destroying thelinearity of the signal over the major portion of the lm range, whilethe record made with the invention is reproducibley with the normalreproducing system.

The principal object of the invention, therefore, is to improve thevariable area type ofI sound recording.

Another object of the invention is to record sound in aY variable areatype of sound record over a wide range of levels without loss of theexpressive dynamics of the original signal.

A further object of the invention is to increase the iilm amplituderange of the soundtrack area of a lm without disturbing the normalrecording or reproducing circuits.

A further object of the invention is to compress a range of 8 db. into afilm width range of 2 db. without any substantial loss inintelligibility when normally reproduced.

Although the novel features which are believed' to be characteristic ofthis invention are pointed out with particularity in the claimsappended' herewith, the manner of its organization and the mode of itsoperation will be better understood by referring to the followingdescription read in conjunction with the accompanying drawing forming apart thereof, in which Fig. I is a diagrammatic view of a variable areasound recording'system embodying the invention;

Fig. 2 is an elevational view of the aperture mask employed in Fig. 1;

Fig. 3 is a view showing the specic details of the mask of Fig. 2; and

Fig. 4 is a graph showing what occurs to a signal recorded with thesystem of Fig. 1.

Referring now to Fig. 1, light from a constant light source 5 iscollected by a lens 6 and projected to an aperture mask 1 havingapertures 8 and 9 therein. The light passing through the apertures 8 and9 is controlled by a pair of shutters II operated by an electromagneticcoil I2 connected to a noise reduction unit 4 over conductors 3.

The light beams passing the apertures 8 and 8 and the shutters areprojected by a lens I4 on a galvanometer mirror I5 actuated by anelectro-magnet I 6 connected to an amplier 2 over conductors I3, which,in turn, is connected to a signal source I through amplifier Thereilected light from the mirror I is projected to a slit mask I1 havinga slit I8 therein, the light beams being shown on the mask I1 as shadedportions I9 at times of no signal. The light passing to slit I8 isprojected by objective lenses 2| to the sound track portion 22 of a lm23. This sound recording system just described is similar to the wellknown variable area sound recording system as disclosed in Patents2,102,776, 2,102,777 and 2,102,778.

Referring now to Fig. 2, the general arrangement of the mask 1 is shownwherein the apertures 8 and 9 are shown in detail as having straightportions 25 and vcurved portions 26 and 21. portions of the apertures,reference is made to Fig. 3 wherein the slit I8 is shown with respect tothe straight portion 25 and curved portions 26 and 21 of aperture 8, forexample. Only one aperture is illustrated in Fig. 3 since the otheraperture has identical dimensions, the two apertures producing the wellknown standard duplex track.

Assuming the slit I8 has a denite Ilength representing the full-trackrecording width, it is to be noted that the straight section 25 is at anangle of 26 and 34' with respect to the slit and crosses the slit at itscenter. It will also be noted that the curved ends are drawn on a radiusof .093 inch. Thus the light beam may be vibrated or oscillated normalto the slit the distance from the center line of the slit to the extremeends of the curves, which distance, shown in the practical example is.1094 inch. Thus linear modution occurs over a light beam movement'of.0428 inch, while the compression portion represented by the curves 26and 21 permits a light beam movement of another .0666 inch, thesedimensions being preferable for the usual optical arrangement ofstandard variable area recording systems.

Considering this particular aperture from the track width standpoint,the linear modulation exists over 80% of the track width whichcorresponds, from the signal amplitude standpoint, to l For the detailsof the curvedand linear of the normal lm range, However, with theparticular curvature shown, the last 2 db. of film track range will nowaccommodate a 6 db. increase in signal level beyond normal linearitywhich would correspond to a track width extending to the points y, asshown by the extensions of the straight portion 25. To bring about thisrelationship, it was found necessary to make the distance b, which isthe distance between the points x, substantially equal to the distancea-f-a', as shown in Fig. 3, this amount of compression beingparticularly suitable to prevent listener annoyance at theatre levelsand obtain the flexibility of recording mentioned above. It also permitsreproduction of the record without expansion as will now be explained.

To illustrate exactly what occurs to a signal beingrecorded with thesystem of Fig. 1, reference is made to Fig. 4, wherein two cycles of aine wave are shown, the first cycle 38 having an amplitude whichnormally would come just within the upper limit of the sound track areaas shown at 3|. Because of the curvatures 26 and 21 of the slit, thepeak of cycle 30 or other waves of this amplitude will be slightlyflattened, as shown by the dot-and-dash line 36, this amount offlattening being insuilicient to effect intelligibility. Furthermore,the major portion of any recorded signal lies below the line 35 so thatvery little of the signal will suffer any distortion whatsoever.

. Now, referring to the second cycle, it will be seen that this cyclehas an amplitude which would normally extend considerably beyond the lmwidth, as shown by the dotted line 32 and which might represent pistolshots or dialogue dramatics. By the invention, this peak is broughtwithin the lm range by attening, the peak as shown by the broken line 33between the lines 3| and 35. If the cycle were recorded normally, thetop of the peak would appear as shown by the at top 34, which destroysintelligibility to a much greater extent than the shape 33. Furthermore,the wave form 33 is reproducible at a l5 or 20 db. greater level withoutappre-` ciable annoyance whatsoever as compared to the square-toppedwave 34. Thus, by the application of applicants particularly curvedaperture, as illustrated, he has retained the desirable linearrelationship between signal variation and light variation over the majorportion of the film track range and signal variations, and has broughtsubstantially the entire range of energy levels of the signal within thelm width in a manner to reduce annoyance and improve the general overallefficiency and quality of the system. He has permitted the soundrecording operator a greater latitude, produced a more flexible system,minimized the defects of a signal source, and permits reproduction ofthe record with the normal reproducing system.

Although one specific aperture has been described above, it is to beunderstood that for other types of signals, other aperture forms may bepreferable.

I claim as my invention:

1. A sound recording system comprising a light source, means for forminglight from said source into a beam of a predetermined shape, means forpassing a portion of said beam to a film, and means for varying theamount of said portion of said beam passed to said film in accordancewith sound waves to be recorded, said light beam forming means providinga linear relationship between the amplitude of said sound waves and theamount of said beam passing to said lm over a, range of signalamplitudes of said sound waves corresponding to 80% of the nlm range anda non-linear relationship between the amplitude of said sound Waves andthe amount of said beam passing to said film over a higher range ofamplitudes of said sound waves. said j higher range of amplitudes beingrecorded in the remaining 20% of lm range.

2. A sound recording system comprising alight source, means forprojecting light froml said source to a film, said film having apredetermined width of sound track area, an aperture for forming saidbeam into a substantially triangular shape, and means for vibrating saidbeam in accordance with sound waves to be recorded, the position of saidbeam determining the amount of light passing to said iilm, said beamhaving one side thereof with a straight portion to pass light to saidfilm in direct proportion to the amplitude variations of said soundwaves and with a curved portion to increasingly decrease the light tosaid iilm per unit increase in amplitude of saidl sound waves, saidstraight portion of said triangular beam corresponding to substantially80% of said predetermined Width lof the sound track area of Said 3. Asound recording system comprising a light source, a lm, means forforming a slit, and means for projecting certain amounts of said lightto the sound track area of said lm through said slit in accordance withthe amplitude of sound waves to be recorded, said means including a maskhaving a substantially triangular aperture, one side of said aperturehaving a straight portion corresponding to substantially 80% of saidsound track area Width, and a curved portion corresponding to theremaining 20% of said sound track area width.

4r A sound recording system in accordance with claim 3 in which thestraight portion of said aperture lies at an angle of substantially 2634with respect to said slit.

5. A sound recording system Ain accordance with claim 3 in which thecurved portion of said aperture is tangent tosaid straight portion andhas' a radius of substantially .093 inch.

6. A light beam defining aperture mask for a sound recording systemcomprising a substantially triangular aperture having two straight sidesand one side partially straight and partially curved, the curved portionbeing tangent to said partially straight portion and having a radius ofsubstantially .093 inch.

7. The method of sound recording comprising linearly varying the amountof light reaching substantially 80% of the sound track recording area ofa lrn in accorda-nce with the amplitude of sound waves to be recordedand continuously decreasing the variations in light reaching the lm withincreasing sound wave amplitudesr over the remaining 20% of said soundtrack area.

8. The method of sound lm recording which comprises compressing intosubstantially the upper 2` db. range of the sound track, 8 db. of thesignal range, 6 db. of said 8 db. normally lying beyond the upper soundtrack limit, and linearly recording over the remaining range of saidsound track extending to the lower limit of the film range.

9. The method of sound film recording within a width of sound track areaapproximating a 30 db. range which comprises compressing intosubstantially the upper 2 db. range of said sound track width, 8 db. ofthe signal range, and linearly recording over the remaining range ofsaid sound track width.

CECIL N. BATSEL.

